Expandable device having bistable spring construction

ABSTRACT

The present invention is directed to bistable cells and their use in devices, particularly medical devices such as stents, clamps and valves. An expandable stent formed of a plurality of bistable cells is described. The stent has two or more stable configurations, including a first stable configuration with a first diameter and a second stable configuration with a second, larger diameter. A valve comprising a bistable cell for use in eliminating incontinence is also disclosed.

BACKGROUND OF THE INVENTION

[0001] There are several kinds of stents on the market with eitherballoon expandable or self expanding function. Balloon expandable stentsare generally made from a material that can easily be plasticallydeformed into two directions. Before insertion, the stent is placedaround the balloon section at the distal end of a catheter and pressedtogether to reduce the outer dimensions.

[0002] As soon as the stent is brought into the body in the proper axialposition it can be expanded and thereby plastically deformed by pumpingup the balloon. In this final position, the stent is at its largestdiameter and should function to support the surrounding tissue,preventing an undesired shape change into a much smaller diameter, atleast locally.

[0003] Therefore, the stent needs to have sufficient rigidity in theradial direction, but also some flexibility in the axial direction whenit is in the final position. Further, the amount of material should beas small as possible and in the inner surface of the stent should notobstruct the flow through the channel (e.g., for blood) or cause toomuch turbulence.

[0004] Problems that generally occur with these stents are as follows:After compressing the stent to its smallest diameter around the balloon,the stent will always have some elastic spring back to a sightly largerdiameter, which can cause problems when the catheter is brought into thepatient's body. In addition, the axial friction between balloon andstent can become so small that the stent slips off the catheter.Further, a larger size stent is typically a disadvantage.

[0005] A further problem is the so called recoil of these stents. Thismeans that after expansion by the balloon pressure, the outer diameterwill always become slightly smaller as soon as the balloon is deflated.This degree of recoiled can be as much as 10%, which can cause migrationof the stent.

[0006] A different type of stent is made of a more or less elasticallyexpanding structure, which has to be held on the catheter by someexternal means. An example of this type is a stent that is held in itsconstrained state by a delivery sheath, that is removed at the momentthat the stent should deploy to its natural form.

[0007] Some of these stents are made of shape memory material witheither superelastic behavior or temperature sensitive triggering of theexpansion function.

[0008] A disadvantage of these self-expanding stents is the need for thedelivery sheath, causing a larger insertion diameter. The removal of thesheath also requires a sheath retraction mechanism, which has to beactivated at the proximal end.

[0009] Most stents of both types further have the disadvantage ofrelatively large length change during expansion and a poor hydrodynamicbehavior because of the shape of the metal wires or struts.

[0010] Another disadvantage of some stents is the positive spring rate,which means that further expansion can only be achieved by higherballoon pressure.

[0011] The construction of prior stents is typically made in such a waythat the external forces, working on the stent in the radial direction,merely cause bending forces on the struts or wires of the structure.

[0012] For example, a unit cell of a Palmaz-Schatz-stent, as produced byJohnson & Johnson Interventional Systems or the ACT One Coronary stent,produced by Progressive Angioplasty Systems, Inc. has in its collapsedcondition a flat, rectangular shape and in its expanded condition a moreor less diamond-shaped form with almost straight struts (Palmaz-Schatz)or more curved struts (ACT-One).

[0013] The shape of the unit cell of such stents is typicallysymmetrical with four struts each having the same cross section. Inaddition, the loading of the cell in the axial direction will typicallycause an elastic or plastic deformation of all of the struts, resultingin an elongation of the unit cell in the axial direction. These unitcells have a positive spring rate. In stents based upon these unit cellsthe stability against radial pressure is merely dependent on the bandingstrength of the struts and their connections.

SUMMARY OF THE INVENTION

[0014] In this patent application a new type of stent is described witha unit cell, having a negative spring rate and a bistable function. Sucha unit cell can also be used in other medical applications. This meansthat it has two configurations in which it is stable without the needfor an external force to hold it in that shape. The unit cell is formedusing at least two different sections. One section is less pliable thanthe other one and acts a relatively rigid support that hinders the shapechange of the more pliable section in one direction. In the otherdirection the pliable section can be deformed, but because of theopposing force from the rigid section, the stability of the pliable orflexible section is strongly increased.

[0015] External forces in a direction perpendicular to the most pliablesection are distributed to the rigid section and the cross section ofthe pliable section is merely loaded in compression mode. This makes theconstruction much stronger than prior stents. In prior stents, allstruts have generally the same cross section and mechanical propertiesand are merely used in the bending mode.

[0016] The construction of a stent, based upon this unit cell results inan apparatus, that can easily be elastically compressed around theballoon by finger pressure.

[0017] Below a certain critical diameter, the present stent snapsfurther to a stable, smallest diameter, thus holding the deflatedballoon firmly on to the surface of the catheter, with an insertiondiameter that is as small as possible. An additional sheath is notrequired, but may be used for extra safety.

[0018] After the stent has been brought into the patient's body at theproper axial position, the balloon can be inflated until the stentreaches its critical elastic equilibrium diameter. Slightly above thisdiameter the stent automatically expands further to its final largestdiameter, where it reaches its maximum stability against radialpressure. The design enables a constant length large expansion ratio, areliable expandability and/or a small surface ratio.

[0019] A further embodiment of this invention is the possibility of akind of stepwise expanding stent with a range of stable diameters.

[0020] Another part of the invention is a stent with several externaldiameters along its length, to adapt as good as possible to the shape ofthe body cavity where it is placed.

[0021] Another part of the invention is the possibility to modify thestress and strain pattern in the unit cell by means of a heat treatmentin such a way, that the force displacement characteristic of this unitcell becomes asymmetrical or even exhibits a monostable instead of abistable function, either with the expanded diameter or the collapseddiameter being the most stable condition.

[0022] Another embodiment of the invention is the modification of thegeometry of the cross section of some struts to achieve the symmetric orasymmetric bistable or monostable force against displacementcharacteristics of a unit cell.

[0023] Another part of the invention is the use of one or more unitcells in other medical applications such as, for example, an expander ora clip, either to spread a body cavity open or to clamp or hold a bodypart or some body tissue.

[0024] The invention is also directed to the use of the inventive stentsin conjunction with inventive expander rings to join together twovessels.

[0025] The invention is also directed to a bistable valve having asnap-action bipositional unit cell and uses for the same, in particular,to prevent incontinence.

[0026] The invention is also directed to multistable cells and their usein medical devices.

[0027] Description of the Construction.

[0028] The construction of the present stent includes a series ofelements with an arrangement of unit cells that enable the stability ina special way. Each unit cell exists out of at least two distinct,mechanically connected sections with different mechanical behaviors. Onesection acts as a relatively rigid support for the more flexiblecounteracting section. The more flexible section is responsible formost, if not all, of the expansion of the stent. There are several waysto manufacture a stent based upon this principle and it can be made fromseveral materials, like polymers, composites, conventional metals orshape memory alloys with superelastic behavior or with temperaturesensitive behavior.

[0029] It can be made from an arrangement of wire or strip, weldedtogether at specific places. Another possibility is metal deposition inthe desired pattern onto a substrate or the use of sintering ofprealloyed powder.

[0030] A further method is making the stent from a tubular shapedstarting material, with a pattern of slits or slots made in the wall bymeans of etching, grinding, cutting (e.g., with a laser, water, etc.),spark erosion or any other suitable method. The pattern can also be madein a flat plate and then welded, brazed or crimped to a more or lesscylindrical shape or a cylindrical mid section with two conical endswith larger diameters.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 shows the principle of a bistable mechanism;

[0032]FIG. 2 shows the force-displacement characteristic of themechanism of FIG. 1;

[0033]FIG. 3 shows another bistable mechanism with an asymmetricbistability;

[0034]FIG. 4 shows the force-displacement characteristic of themechanism of FIG. 3;

[0035]FIG. 5a shows an inventive tubular stent in the stable, fullycollapsed configuration;

[0036]FIG. 5b shows an inventive tubular stent in the stable fullyexpanded configuration;

[0037]FIG. 6 shows a part of a stent with one bistable unit cell, drawnin the stable expanded shape;

[0038]FIG. 7 shows the part of the stent of FIG. 6 near its elasticbistable equilibrium position;

[0039]FIG. 8 shows the part of the stent of FIGS. 6 and 7 in its stablecollapsed shape; and

[0040]FIG. 9 shows a larger section of the stent of FIGS. 6 and 8,showing some unit cells in the collapsed shape and some unit cells inthe expanded shape.

[0041]FIG. 10 shows an inventive stent formed of a plurality of smallerinventive stents joined together with flexible connectors.

[0042]FIG. 11 shows a partially expanded inventive stent having morethan one type of bistable unit cell;

[0043]FIG. 12 shows an inventive stent having a range of diameters alongits length;

[0044]FIG. 13 shows an inventive expansion ring in expanded state;

[0045]FIG. 14 shows the expansion ring of FIG. 13 in contracted state;

[0046]FIG. 15 shows an inventive stent joining two vessels together andfurther secured with inventive expansion rings, the stent exterior tothe vessels;

[0047]FIG. 16 shows a cross-sectional view of FIG. 15 along section line16-16;

[0048]FIG. 17 shows an inventive stent joining two vessels together, thestent interior to the vessels;

[0049]FIG. 18 shows two vessels joined together with an inventiveexpansion ring and a clamp

[0050]FIG. 19 shows a bistable valve in the closed position;

[0051]FIG. 20 shows the bistable valve of FIG. 19 in the open position;

[0052]FIG. 21a shows a multistable cell in the fully contracted state;

[0053]FIG. 21b shows the multistable cell of FIG. 21a in the fullyexpanded state;

[0054]FIG. 22a shows another multistable cell in the fully contractedstate;

[0055]FIG. 22b shows the multistable cell of FIG. 22a in the fullyexpanded state;

[0056]FIG. 23 shows several unit cells as shown in FIGS. 21a,b joinedtogether and in the fully expanded state;

[0057]FIG. 24a shows several unit cells as shown in FIGS. 22a,b joinedtogether and in the contracted state;

[0058]FIG. 24b shows the interconnected cells of FIG. 24a in fullyexpanded state;

[0059]FIG. 24c shows the interconnected units cells of FIG. 24a in theprocess of expanding; and

[0060]FIG. 24d shows several strips of interconnected cells as in FIGS.24a,b joined together and in the process of expanding.

DETAILED DESCRIPTION OF THE DRAWINGS

[0061]FIG. 1 shows the principle on which the stent is based, FIG. 1ashows a rod 1 with a length L, which is compressed in its axialdirection unit; it reaches its buckling stress. Then the central part ofthe rod will band out in a sidewards direction, either to position 2 or3 (dashed lines in FIG. 1b). When the axial displacement L of the endsof the rod is held stable by external clamps 4, it is possible to movethe central section of the rod between the two stable positions 2 and 3.This movement is in a direction X, perpendicular to the original lengthaxis A-A of the rod. All positions between the stable positions 2 and 3are unstable. In FIG. 1b the central part of the rod has to rotate overan angle β before the rod can be moved in direction X. FIG. 1C shows asecond order curvature in rod 1, which occurs when the rotation overangle β is opposed by clamping the central part of rod 1 and maintainingthis part parallel to the axis A-A.

[0062]FIG. 2 shows the force F as a function of displacement X, with Xdisplayed in the horizonal direction. The rod is moved from the upper 2to the lower 3 stable position of FIG. 1. The force increases rapidlyfrom zero to Fmax. At that moment the onset of either the first orsecond order curvature of FIGS. 1b and 1 c is reached. Furtherdisplacement in direction X costs less force, because this spring systemhas a negative spring rate. The force even becomes zero in the midposition and further movement occurs automatically. It can be seen inFIG. 2 that the system is completely symmetrical and the force needed tomove back from the lower to the upper position has the samecharacteristic.

[0063]FIG. 3 shows rod 5, which will have an asymmetrical forcedisplacement characteristic, because it already has a preset curvature,even in the unloaded position, where the length is already L-L. This canbe achieved by prior plastic deformation, heat treatment or the use ofan asymmetrical geometry of the cross section of the rod (not shown).The rod 5 in FIG. 3 can be mounted between two clamps on a length L-L,and if it is elastically deformed in the same way as the rod in FIGS. 1band 1 c, it will have a different stress distribution in the crosssection in end position 2 and 3, compared to the rod of FIG. 1. Thismeans that the rod has become a preferent unloaded/table position, shownin FIG. 3.

[0064]FIG. 4 shows the asymmetrical force-displacement characteristic ofthe precurved rod of FIG. 3. The initial displacement form the stableupper position needs a starting force F1 and if the rod is in its tablelower position the starting force in the opposite direction is only F2,being smaller than F1. Force F2 can be made as small as desired, evenzero or negative, but needs to have a positive value if stability of thelower position is required.

[0065]FIGS. 5a and 5 b show the general appearance of an inventivetubular stent in fully contracted and fully expanded configurationrespectively. The stent, in its fully contracted state shown generallyat 50 and in its fully expanded state shown generally at 60, iscomprised of a plurality of interconnected bistable unit cells (shown inthe expanded state at 64 in FIG. 5b). The bistable unit cells are formedfrom a first relatively rigid segment 52 (66 in FIG. 5b) and a secondrelatively flexible segment 54 (68 in FIG. 5b), joined together at ends70 and 72. Second relatively flexible segments 68 are interconnectedwith adjacent relatively rigid members 66. Adjacent cells in thelongitudinal sense (the longitudinal axis is denoted by referencenumeral 75) are joined at ends 70 and 72. By applying a uniform radiallyoutward or inward force, the stent may be switched directly from a fullycontracted to a fully expanded configuration or vice versa.

[0066]FIG. 6 (corresponding to inset 6 in FIG. 5b) shows a small part ofa stent such as that shown in FIG. 5 which uses the bistable function ofa unit cell, according to the present invention. The drawing shows ahorizontal line A-A, which is parallel to the central axis of the stent.There are two series of sinusoidal segments with distinct size (see alsoFIG. 9 for an overview). The segments 7 and 9 have a relatively largecross section. Only segment 9 is shown entirely. The segments 9 and 10have a relatively smaller cross section, and here only segment 8 isentirely shown. The segments are interconnected for example welded, atjoints 11 and 12.

[0067] Because of the difference between the cross section of segment 8and 9, the deformation force of segment 8 is much lower than for segment9. Therefore, segment 9 can be considered as a relatively rigid clamp,like the clamps 4 in FIG. 1b opposing relative displacement between thejoints 12 in the axial direction, parallel to axis A-A. In contrast,segment 8 acts as a flexible rod, like rod 1, described in FIG. 1 or rod5, described in FIG. 3. This combination of segments 7 and 8 or 9 and 10defines a unit cell, acting as a bistable spring system with aforce-displacement curve F-X like the described curves of FIGS. 2 and 4,depending on the unloaded condition and geometry of the segments.Alternatively, instead of using segments or struts of differentdiameter, the segments can have the same diameters (i.e., crosssectional area) and exhibit different strengths or rigidity and stillaccomplish the same effect. One way to obtain such differences instrength or rigidity would be to use different materials for thesegments. Another way would be to use the same material, like a metal,for all the segments but selectively strengthen (e.g., by heat treating)those segments that need to be rigid. It should be noted that heattreatment will not strengthen all materials. Nitinol, for examplebecomes more pliable as a result of heat treatment. This property ofNitinol can be exploited, however, to render one section of Nitinol morepliable relative to a second, non-heat-treated section of Nitinol.

[0068]FIG. 7 shows the same part of the stent (as depicted in FIG. 6)near the elastic equilibrium position. Segment 8 has bene deformed intothe direction X, caused by force F, but segment 9 has almost itsoriginal shape, because of its larger rigidity.

[0069]FIG. 8 shows the same unit cell of the stent of FIGS. 6-7 after ithas been pressed through the elastic equilibrium position. Itautomatically snaps into its stable position of FIG. 8. This snappingforce can be strong enough to hold a deflated balloon tight on thecatheter shaft (not shown), depending on the mechanical characteristics(e.g., the strength) of the material(s) used to make the segments. Withthe geometry shown in these figures, the segments 8 and 9 fit closetogether, taking up a minimum amount of space when the stent is in itssmallest stable diameter.

[0070]FIG. 9 shows a section of the stent of FIG. 5, flattened forillustrative purposes, showing several flexible segments in thecollapsed stable shape (segments 14, 18 and 20) and one segment element16 in the expanded stable shape. Segments 13, 15, 17, and 19 arerelatively rigid segments and substantially maintain their originalshape. The distance between two relatively rigid segments is shown as(h) in the collapsed stable shape and (H) in the expanded stable shape.The value of the displacement (H-h) in the direction X depends on theheight of an expanded unit cell or amplitude of the segments and thesize of the connecting joints. The described part of the stent is shownas a flat surface, but it may be clear that a cylindrical stent such asthat shown in FIG. 5 is shaped if segments 13 and 20 are directlyconnected to reach other with joints 21. In other words, the stent isshown separated along the joints 21 and in a flattened condition.

[0071] The range of stable diameters of the stent changes with the value(H-h)/π, each time that a flexible segment snaps from the collapsedstable position to the expanded stable position. The result is a stentwith an extremely rigid surface at all diameters being able to withstandthe external forces better than with conventional stents. In the lengthdirection, the flexibility of the stent can be increased bydisconnecting several unit cells from their neighbor unit cells, forexample, by cutting the center of one or more joints while maintainingthe several joint pieces as joints.

[0072] Another method to increase flexibility is to change the geometryof several sections of the unit cells in the length direction from therelative flexible to the relative rigid shape several times along thetotal length of the stent. In other words, referring to FIG. 9 one ormore or each of the segments 13-20 could be constructed with larger andsmaller diameter (or otherwise flexible and rigid) sections whichalternate after each joint 21.

[0073] Another possibility, as shown in FIG. 10 is the use of a seriesof short multistable stents 100 aligned lengthwise end to end andconnected with flexibility joints 104 having the same or a differentgeometry or configuration as the joints forming individual unit cells.

[0074] The scope of the invention should include all types of material.One of the most interesting materials is superelastic Nitinol, becauseof its large elastic strain, well defined stress values, caused by theirplateau stresses and the possibility to define the desired curvatureinto the metal by means of a heat treatment. A stent of Nitinol can bemade by forming slits or slots in a tube, while in its collapsed orsmaller stable diameter. The slotted tube is then expanded by a separateshaping tool and heat treated on this tool to define the expanded stablediameter as the unstrained shape.

[0075] In a more general sense, the present invention is directed to adevice having a plurality of stable configurations. The device iscomprised of a plurality of interconnected multistable cells. The cellsinclude one or more relatively rigid sections and one or more relativelyflexible sections interconnected so as to define a cell structure in theform of a multistable spring system having a plurality of stableconfigurations. In a preferred embodiment, the cells comprise a firstarcuate member having first and second ends and a second arcuate memberhaving first and second ends, the first end of the first member incommunication with the first end of the second member, and the secondend of the first member in communication with the second end of thesecond member. It should be noted, however that members need not berigorously arcuate. Other shaped members, including relatively straightmembers are contemplated as well.

[0076] The invention, in particular, contemplates bistable cells, thatis cells having two stable configurations. In one such cell, thedistance between corresponding points on the first and second sectionsis larger in the first stable state of the cell than in the secondstable state of the cell. The cells themselves are constructed andarranged so that the device itself is at least bistable and possiblymultistable. One such device, a cylindrical stent having two or moreconfigurations with an initial diameter size and a final larger diametersize has been described above. However, mutistable stents are alsocontemplated. Thus, for example, a stent may be constructed in which thecells are designed and arranged to provide a range of diameters instep-wise fashion. One such way this may be accomplished would be toemploy several different types of cells in the stent, each type of cellhaving a different spring constant so that depending on the amount offorce used, the stent would assume a different diameter. Such a stent ina partially expanded state is shown schematically in FIG. 11. Apartially expanded stent is shown generally at 120. The stent iscomprised of relatively rigid segments 123, 127, 131 and 135 whichsubstantially maintain their original shape, and relatively flexiblesegments 125, 129, and 133. The segments are interconnected, with joints122. As depicted, first flexible elements 125, and 133 are in anexpanded configuration while second flexible element 129 is in acontracted configuration. By applying a radially outward or tangentialforce, flexible element 129 may be flipped to its fully expandedconfiguration resulting in a stent (not shown) with a larger diameter.As shown in FIG. 11, cells 138 are larger than cells 136 even in thecontracted state. First flexible elements 125 and 133 are characterizedby a different degree of flexibility than second flexible element 129.

[0077] Another form of stent, as shown generally at 150 in schematicFIG. 12, has an first diameter at a first end 152, a second diameter ata second end 154 and one (or more) intermediate diameters in a region156 between first end 152 and second end 154, the intermediate diameterdiffering from the first and second diameters. The interconnected cellsin such a stent, as shown generally at 150 in rig. 12 may all have thesame force constant and hence be openable all at once with theapplication of the necessary force or there may be several differenttypes of cells, each with their own force constant In order to achievethe multiplicity of diameters, cells of differing sizes may be used. Inone embodiment of this type of stent, the first and second diameters arethe same while in another embodiment, the first and second diametersdiffer.

[0078] The present invention is also directed to a method of implantingan expandable stent having a plurality of stable configurations. Themethod comprises the steps of applying the stent to an expanding meanson a catheter, delivering the stent to a desired bodily location,expanding the expanding means so as to expand the stent from a firststable configuration to a desired second stable configuration, thesecond stable configuration having a larger diameter than the firststable configuration, and deploying the expanded stent at the desiredbodily location. The expanding means may be a balloon, a mechanicaldevice on or in the catheter, a heat source where the cells can beinduced to change states by heating or any other suitable expandingmeans. The stent may be applied to the balloon in the first stableconfiguration or may be applied in the second stable (expanded)configuration during the applying step. In the latter case radiallyinward pressure may be applied to the stent so as to urge the stent intothe first stable configuration to snap it onto the catheter. Where thestent has additional stable states, the stent may be applied to theballoon in an intermediate stable state in which the diameter of thestent is intermediate between the diameter in the first state and thediameter in the second state. Again, the stent may be locked on theexpanding means by further applying a radially inward pressure.

[0079] A further object of the invention is the use of a single bistableunit cell as an expander (expansion ring), that can be brought into anarrow place and then triggered to snap back into its expanded stableshape. As shown in FIG. 13 an expansion ring shown generally in itsexpanded state at 250 consists of a first rigid member 254 having first258 and second 262 ends and a second more flexible member 266 havingfirst 270 and second 274 ends. First end 258 of first member 254 isconnected to first end 270 of second member 266 and second end 262 offirst member 254 is connected to second end 274 of second member 266.FIG. 14 depicts the expansion ring of FIG. 13 in its contracted state.Second member 266 is seen to be in a second stable position.

[0080] Another object of the invention is the use of a single bistableloop (unit cell) as a clip, that can be used to clamp on an artery,fallopian tube or any other body part, to close or hold it for sometime. For such a clip it may be desirable to define the collapsed stableshape as the unstrained shape, because the collapsed stable shape has tobe the most stable one. In the collapsed state, the clip would resemblethe collapsed expansion ring of FIG. 14. A triggering means would beused in conjunction with the clamp to switch the bistable loop from onestate to another. The triggering means may be pneumatic, hydraulic,mechanical, thermal or electromechanical means. Examples of suchtriggering means include a human hand applying force to the bistableloop, and the application of heat to the loop. Other triggering meansinclude pulling on the device, pushing on the device, bending the rigidsection of the device or release a restraint holding the flexible memberin place.

[0081] Another part of the present invention involves constructionsbetween one or more ring-shaped elements according to the presentinvention, combined with a tubular sleeve that is reinforced or heldopen with such elements. An example is a so-called graft stent made of apolymer with one or more expansion rings. The expansion rings mayconsist of the above-described bi-stable cells. The surface of the stentcomprises a skin mounted on the expansion rings. In mounting the skin,the skin may surround, be in or between the expansion rings. The skinmay be human or animal skin, a polymeric material or any other suitablebio-compatible material. Such a stent may comprise one or more expansionrings, such as a first expansion ring at a first end of the stent and asecond expansion ring at a second end of the stent. The stent may be ofconstant diameter along its length or may have a first diameter at thefirst end and a second diameter at the second end.

[0082] The present invention is also directed to a stent having anunexpanded configuration and an expanded configuration, and comprising aplurality of generally longitudinal, wave-like first memberscharacterized by a first wavelength, and having peaks and troughs and aplurality of generally longitudinal wave-like second memberscharacterized by a second wavelength, and having peaks and troughs. Thewavelengths of the first and second longitudinal members aresubstantially equal. The second members are capable of stably assumingtwo positions, a first position corresponding to the unexpandedconfiguration in which the first and second members are in phase and asecond position corresponding to the expanded configuration, in whichthe first and second members are 180° out of phase. The first membersare more rigid than the second members. The first and secondlongitudinal members are disposed on the surface of the stent such thatthe longitudinal first and second members alternate. In the unexpandedstate, each peak of each first member is connected to one adjacent peakof a second member in a region of attachment and each trough of eachfirst member is attached to one adjacent trough of a second member in aregion of attachment, as can be seen from FIG. 8. The regions ofattachment are separated along the longitudinal direction by onewavelength. The so described stent can be snapped from the unexpandedconfiguration to the expanded configuration by applying a radiallyoutward force and similarly can be snapped from the expanded to theunexpanded configuration by applying a radially inward force. While suchstents may be used internal to a bodily vessel, it may also be usedexternal to vessels to join two vessels together.

[0083] The invention also contemplates a method of joining together twovessels comprising the steps of delivering an inventive stent in anunexpanded configuration in a first stable state to a bodily site,expanding the stent to a second stable state, the diameter of the stentin the second stable state exceeding that of the vessels to be joinedand placing the stent over the vessels to be joined. The stent may thenbe contracted to a third stable state, the stent in the third stablestate having a diameter intermediate between the diameters of the stentin the unexpanded state and in the second stable state. The stent mayfurther be secured to the vessel with the aid of one or more of theabove-described expansion rings (a bistable loop). One or more expansionrings, such as that depicted in FIGS. 13 and 14 or small clamping stents(such as that formed from the strip shown in FIG. 23) may be deliveredto each side of the stent in a contracted state and deployed so as toclamp the vessels between the ring(s). Multiple rings may be used foradditional clamping. As shown generally at 300 in FIG. 15, a firstvessel 304 and a second vessel 308 are joined together with inventivestent 312. Vessel 304 overlaps stent 312 in a first overlap region 316while vessel 308 overlaps stent 312 in a second overlap region 320.Vessel 304 is clamped between expansion ring 324 (shown in the expandedstate) and stent 312 while vessel 308 is clamped between expansion ring328 (shown in the unexpanded state for illustrative purposes only) andstent 312. the dotted lines associated with expansion ring 328illustrate expansion ring 328 in its expanded state. It should beadditionally noted that FIG. 15 provides a cut-away view of vesselsshowing the rings contained therein. FIG. 16 shows a cross-sectionalview of FIG. 15 along section line 16-16. Vessel 304 is shown sandwichedbetween stent 312 and expansion ring 324.

[0084] In another embodiment, as shown in FIG. 17, a first vessel 404and a second vessel 408 are joined together by a stent 412. First end416 of stent 412 rests in vessel 404 while second end 420 of stent 412rests within vessel 408. Optional clamps (such as a small portion of acollapsible inventive stent shown later in strip form in FIG. 23) 424and 428 residing on the outside of vessels 404 and 408 clamp the stentto the vessel. Additional clamps may be used as needed.

[0085] In another embodiment, a combination of the embodiments of FIGS.15 and 17, the first end of the stent may protrude from one of thevessels and the second end of the stent may extend over the secondvessel. Again, clamps and expansion rings may be used to further securethe stent to the vessels.

[0086] In another embodiment, as shown in FIG. 18, vessel 454 and vessel458 are held together by an expansion ring 462 internal to the vesseland a clamp 466, consisting of, for example, a small section ofcollapsible stent, the stent chosen so that the diameter of the stent ina collapsed state affords a snug fit with vessels 454 and 458 andexpansion ring 462. Either the expansion ring or the clamp, but notboth, may be replaced by a suitable support such as a rigid collar.

[0087] The invention also contemplates a method of joining together twovessels comprising the steps of delivering an inventive stent in anunexpanded configuration in a first stable state to a bodily site,placing two bodily vessels over the stent and expanding the stent to asecond stable state, the diameter of the stent in the second stablestate exceeding that of the vessels to be joined. The diameter of thestent in the second stable state is preferably chosen so that thevessels fit snugly over the stent. The delivery of the stent may beaccomplished by delivering the stent in an unexpanded configurationthrough a bodily vessel and subsequently expanding the stent to restsnugly in the vessels to be joined (where a portion of the stent residesin a vessel), or by expanding the stent to its most expanded state,placing the stent over the vessel and then contracting the stent to anintermediate state over the vessel. The collars and expansion ringsmentioned above may similarly be delivered. Alternatively, the stent,collars and expansion rings may be delivered by surgically exposing thevessel in question.

[0088] The present invention is also directed to a bistable valve. Thevalve, as shown generally at 600 in FIG. 19 includes a snap-actionbipositional unit cell shown generally at 604 located within a conduit606. Snap-action bipositional unit cell 604 consists of a (substantiallyarcuate) flexible member 608 having a first end 612 and a second end616. First end 612 is in communication with a triggering means 620 whichis supported, in turn by a support means 624 emerging from the innersurface of conduit 606. Second end 616 of flexible member 608 isanchored to stop surface 628 which extends across conduit 606. Supportmeans 624 and stop surface 628 must be sufficiently rigid to holdflexible member 608 in place and must be more rigid than flexible member608. Stop surface 628 extends substantially obliquely across conduit 606in oblique regions 630 and has a opening 632 within in longitudinalregion 634 to allow the flow therethrough of a fluid. Although opening632 is oriented along the longitudinal axis 636 of conduit 606, those ofordinary skill in the art will recognize other possible orientations ofthe opening and stop surface. Valve closure member 640, actuated betweenopen and closed positions by flexible member 608, is constructed andarranged so as to block the flow of fluid through opening 632 whenflexible member 608 is in the closed position. When flexible member 608is in the open position, as depicted in FIG. 20 valve closure member 640no longer obstructs opening 632, thereby allowing the flow of fluidtherethrough.

[0089] While triggering means 620 may be any suitable mechanical,hydraulic, pneumatic, or thermal based trigger known in the art atpresent or in the future, in a preferred embodiment, triggering means620 is a piezoelectric element. In operation, if the piezoelement shownin FIG. 19 at 620 is not activated, valve closure member 640 is closed.Activation of piezoelement 620, as shown in FIG. 20 causes a smallshortening in the longitudinal length (denoted by Y in FIG. 15) ofpiezoelement 620 which in turn releases flexible member 608 from itsfirst position. With member 608 released, valve closure member 640 isfree to open under the pressure transmitted from the fluid. Member 608assumes a second, inverted, position, as depicted in FIG. 20 While thefluid pressure maintains member 608 in its second position, even in theabsence of any fluid, member 608 remains in its second position, asdepicted in FIG. 20 if the triggering is turned off and piezoelement 620assumes its original length. Valve closure member 640 may be closedagain, in the absence of fluid, by a subsequent triggering ofpiezoelement 620 allowing member 608 to transition to its second(closed) position which is the preferred position of member 608. Member608 has been treated to receive a preferred position as shown in FIG. 3.

[0090] The valve depicted in FIGS. 19 and 20 may be applied to medicaland non-medical devices. It is, in particular, an aim of the presentinvention to apply the inventive bistable valve to the control ofurinary incontinence. In a patient with incontinence, the abovedescribed valve may be implanted in the urethra using any suitable meansincluding the use of the above-described expansion rings to clamp thevalve to the urethra. Although the valve in the default state is closed,the valve may be triggered when the bladder is full, to void thebladder. Upon voiding the bladder, the valve may be triggered again toclose it. Another such application is to employ the inventive valve inconjunction with a shunt. The shunt may be activated by triggering thedevice and similarly may be closed by triggering the device.

[0091] Of course the valve may be used in other medical and non-medicalapplications as well.

[0092] In addition to the bistable unit cells disclosed above, bistableunit cells and more generally, multistable unit cells of other shapesare also contemplated by the present invention. FIGS. 21a and 21 b areschematic representations of another embodiment of an inventive hingedmultistable cell in its contracted and expanded states, respectively.The contracted cell, shown generally at 700, and the expanded cell,shown generally at 705, consist of four interconnected relatively rigidmembers. Two side members 709 are connected to opposite ends of topmember 713 via hinges 715. Side members 709 are connected at theiropposite ends to opposite ends of bottom member 717 via hinges 719.Preferably, the hinges are elastic or plastically deformable. The hingesmay be fixedly attached to the side, top and bottom members or may beintegral with these members. In the latter case, the hinges may beformed by removing material from the cell in the region of the hinges sothat the hinges are thinner or have a different geometry from the side,top and bottom members. In the process of transitioning from theexpanded to the collapsed state, bottom member 717 opens slightly. Thecell of FIGS. 21a,b also has two additional intermediate states in whichone or the other (but not both) of side members 709 and top member 713are collapsed downward.

[0093] A hexagonal hinged multistable unit cell is shown schematicallyin FIG. 22a in the collapsed state and in FIG. 22b in the expandedstate. The cell, shown generally at 750, consists of top member 754 andbottom member 758, and upper side members 762. Two upper side members762 are connected to opposite ends of top member 754 via hinges 756.Upper side members 762 are connected to bottom member 758 via hinges768. Bottom member 758 is shaped like a ‘U’ with the two uprights of the‘U’ modified to lie at oblique angles with respect to the bottom part ofthe ‘U’. As with the previously discussed inventive cells, hinges 756and 768 may be elastic or plastically deformable and may be fixedlyattached to the members or integral with the members. The hexagonal unitcell exhibits multiple stable states. In addition to the fully expandedand fully contracted states shown in FIGS. 22a and 22 b, the hexagonalcell can also achieve two intermediate stable configurations in whichonly one of the two upper side members 762 is collapsed inward alongwith top member 754.

[0094] The above described hinged multistable cells may be used in anyof the above discussed applications e.g. to form stents, clamps, clips,expander rings, bistable valves.

[0095] In one such application a ring or stent is formed of the hingedcells of FIGS. 21a and 21 b. As shown in FIG. 23, a series of unit cellsof the type depicted in FIG. 21 are joined together so that the topmember of a cell forms a portion of the bottom member of an adjoiningcell. As depicted, top member 814 of cell 810 forms a portion of bottomelement 818 of cell 820. Similarly, top member 824 of cell 828 forms aportion of bottom element 832 of cell 836. Although the ring or stent inFIG. 23 has been cut for illustrative purposes, the two ends 840 and 844are normally joined together with a portion of lower member 848 of cell852 serving as an upper member for cell 856. The ring so formed has arange of stable stable states including a fully expanded state and afully contracted state. Where the individual cells are made identically,only the fully expanded states may be accessed by the application of auniform radially outward force to the stent in the fully contractedstate. It may serve as a clamp or collar, an expansion ring or a stent.Larger stents may be formed by interconnecting a plurality of suchrings.

[0096] Similar products may also be formed from other multistable unitscells. FIGS. 24a and 24 b illustrate one such possibility schematicallyin which hexagonal unit cells such as those shown in FIGS. 22a, b may bejoined together to form a ring. The top member 884 of each cell 880 isjoined with a the bottom portion 886 or modified ‘U’ shaped bottommember 890. Although shown in strip form in FIGS. 24a and 24 b, end 894can be joined to end 898 to form a ring. The strip of FIG. 24a is shownin fully expanded state in FIG. 24b. Adjacent cells 880 are seen intheir expanded state. For the sake of completeness, the hinges aredesignated 902. FIG. 24c shows one cell 920 in the process of expandingand one already expanded cell 924. The cells 920 and 924 are joined atbottom member 928 and top member 932. Hinges are shown at 936. Multiplestrips may also be joined together so as to form a stent whose length isa multiple of the length of the unit cell. In such a case, upper sidemembers of adjacent cells would be joined together. This is illustratedin FIG. 24d which, like FIG. 24c shows cells 940 in the expanded stateand cells 944 in the process of expanding. Upper side members 948 areshown by dashed lines. Adjacent strips of interconnected cells 952 arejoined together by upper side members 948 as well as by oblique regions956 of bottom members 960.

[0097] It should be noted that the inventive devices of the presentapplication may be use on a temporary basis or on a permanent basis inthe body. Thus, for example, permanent stents and clamps arecontemplated, as are removable stents and clamps.

[0098] It should further be noted that in expanding-some of theinventive multistable cells, there may be components ofexpansion/contraction in a direction perpendicular to the direction ofthe force applied to expand the cells.

[0099] Finally, for the purposes of this application, the term‘multistable’ is intended to include ‘bistable’.

[0100] In the described drawings and text only some examples ofdifferent embodiments have been given. While the stents of the presentinvention can appear similar to prior stents, the mechanical results arecompletely different due to the special combination of a rigid sectionand a more flexible section in the same unit cell. Of course there are,beside the illustrated sinusoidal shape many other possible basic shapesfor the unit cells, with similar characteristic behavior.

[0101] From the above disclosure of the general principles of thepresent invention and the preceding detailed description, those skilledin this art will readily comprehend the various modifications to whichthe present invention is susceptible. It is intended for the coverage ofthe present application to include different geometries, differentconstructions and different combinations of one or more materials toobtain the same basic mechanical behavior as exhibited by the abovedescribed examples.

1-51.(Canceled).
 52. An expandable device comprising one or moremulti-stable loops, the loop having a first state and a second state,the loop encompassing a first area in the first state and a second areain the second state, wherein the device is expanded by applying a forcethereto.
 53. The device of claim 52 comprising a first arcuate memberhaving first and second ends and a second arcuate member having firstand second ends, the first end of the first member in communication withthe first end of the second member, and the second end of the firstmember in communication with the second end of the second member,wherein the second member is more pliable than the first, the secondmember capable of assuming a first stable position and a second stableposition. 54-72. (Canceled).
 73. An expandable device for use in apassageway, comprising: an expandable conduit formed of a wall having aplurality of slots therethrough with at least a portion of the pluralityof slots having an arcuate shape when the expandable conduit is in acontracted state.
 74. The expandable device as recited in claim 73,wherein the arcuate shape transitions to an expanded shape as theexpandable conduit is radially expanded.
 75. The expandable device asrecited in claim 74, wherein the arcuate shape comprises a wave shape.76. The expandable device as recited in claim 73, further comprising adeformable material surrounding an outer surface of the expandableconduit.
 77. The expandable device as recited in claim 76, wherein thedeformable material comprises an elastomeric material.
 78. Theexpandable device as recited in claim 74, wherein the plurality of slotsare formed at least in part by a plurality of pivotal links.
 79. Anapparatus for use in a passageway, comprising: an expandable conduitconfigured for deployment within a passageway, the expandable conduithaving a plurality of expandable cells that may be transitioned betweena contracted state and an expanded state by application of a forcedirected radially outward, wherein during expansion each expandable cellpasses a transition point that allows the force to be decreased duringtransition to the expanded state.
 80. The apparatus as recited in claim79, wherein the expandable conduit comprises a liner.
 81. The apparatusas recited in claim 79, wherein each expandable cell comprises alongitudinal strut and a flexible link.
 82. The apparatus as recited inclaim 81, wherein each flexible link undergoes pivotal motion duringexpansion.
 83. The apparatus as recited in claim 81, wherein eachflexible link is arcuate in form prior to expansion of the expandableconduits.
 84. The apparatus as recited in claim 81, wherein the forcemay be removed once each of the plurality of cells is moved past thetransition point.
 85. An apparatus for use in a passageway, comprising:an expandable device deployable in a passageway, the expandable devicehaving a plurality of expandable cells, each expandable cell comprisinga thin strut that is flexed between a contracted state and an expandedstate without plastic deformation.
 86. The apparatus as recited in claim85, wherein each thin strut is coupled to at least one thick strut thatremains unflexed during transition of the thin strut from the contractedstate to the expanded state.
 87. The apparatus as recited in claim 86,wherein each thin strut is pivotally coupled to at least one thickstrut.
 88. The apparatus as recited in claim 85, wherein the expandabledevice remains at a substantially fixed axial length during transitionfrom the contracted state to the expanded state.
 89. The apparatus asrecited in claim 86, wherein the thickness ratio of the thick strut tothe thin strut is at least 2:1.
 90. The apparatus as recited in claim85, wherein the expandable device comprises a radially expandabletubular member.
 91. A method of applying a radial force against asurface of a passageway with an expandable device, comprising: formingan expandable device with a plurality of cells expandable between acontracted state and an expanded state; selecting the geometry of thecells such that the cells expand autonomously once expanded past atransition point; and radially expanding the expandable device past thetransition point and against a surface of the passageway.
 92. The methodas recited in claim 91, wherein forming comprises expanding theplurality of cells without axial shortening of the expandable device.93. The method as recited in claim 91, wherein forming comprises formingthe expandable device as a tubular member.
 94. The method as recited inclaim 91, wherein forming comprises forming the expandable device as aliner.
 95. The method as recited in claim 91, wherein selectingcomprises selecting a geometry that utilizes a combination of thickstruts coupled to thin struts.
 96. An apparatus for use in a passageway,comprising: an expandable bistable device configured for deploymentproximate a passageway wall, the expandable bistable device having aplurality of bistable cells arranged in a generally tubular shape, eachbistable cell comprising first and second arcuate members, the pluralityof bistable cells being stable in a collapsed configuration and in anexpanded configuration.
 97. The apparatus as recited in claim 96,wherein the first and second arcuate members of each bistable cell areconnected to each other.
 98. The apparatus as recited in claim 97,wherein the collapsed configuration is a first generally tubularconfiguration and the expanded configuration is a second generallytubular configuration having a larger diameter than the first generallytubular configuration.
 99. The apparatus as recited in claim 96, furthercomprising a device able to transport the expandable bistable device toa desired location in the passageway.
 100. The apparatus as recited inclaim 99, wherein the apparatus further comprises a deployment deviceable to initiate expansion of the expandable bistable device from itsfirst generally tubular configuration to its second generally tubularconfiguration.
 101. The apparatus as recited in claim 97, wherein thefirst arcuate member and the second arcuate member each comprises amidpoint and two ends, and further wherein the first arcuate member ismore flexible than the second arcuate member.
 102. The apparatus asrecited in claim 101, wherein the first and second arcuate members aremechanically connected such that the second arcuate member hindersdeformation of the first arcuate member.
 103. The apparatus as recitedin claim 102, wherein the first arcuate member has two stable positions,the first stable position being where the first arcuate member mid-pointis adjacent to the second arcuate member mid-point, the second stableposition being where the first arcuate member mid-point is displacedfrom the second arcuate member mid-point to form a gap between the firstarcuate member mid-point and the second arcuate member mid-point. 104.The apparatus as recited in claim 96, wherein the second arcuate memberhas a greater thickness than the first arcuate member.
 105. Theapparatus as recited in claim 96, wherein the thickness ratio of thesecond arcuate member to the first arcuate member is greater thanapproximately 3:1.
 106. The apparatus as recited in claim 96, whereinthe bistable device further comprises a wrapping attached to the outersurface of the bistable device.
 107. The apparatus as recited in claim106, wherein the wrapping comprises an expandable material.
 108. Theapparatus as recited in claim 96, wherein the bistable device furthercomprises a deformable material attached to the outer surface of thebistable device.
 110. The apparatus as recited in claim 108, wherein thedeformable material comprises an elastomer.
 111. The apparatus asrecited in claim 107, wherein the wrapping is selected to be resistantto environmental conditions encountered in the passageway.
 112. Theapparatus as recited in claim 98, wherein the bistable device in itssecond generally tubular configuration comprises a plurality ofdiameters.
 113. A method of stabilizing an unsupported section of apassageway, comprising: providing an expandable bistable device having agenerally tubular shape that comprises a plurality of bistable cells,each of the bistable cells comprising first and second arcuate members;placing the bistable device at a position in the passageway while in afirst stable state; and radially expanding the bistable device to asecond stable state having a generally tubular configuration withoutsubstantially reducing axial length.
 114. The method as recited in claim113, further comprising attaching a wrapping to the outer surface of thebistable device.
 115. The method as recited in claim 114, whereinattaching comprises attaching an expandable material.
 116. The method asrecited in claim 113, further comprising applying a deformable materialto the outer surface of the bistable device.
 117. The method as recitedin claim 116, wherein applying comprises applying an elastomericmaterial.
 118. The method as recited in claim 113, wherein radiallyexpanding comprises expanding the bistable device to a plurality offinal diameters.
 119. A method for installing a liner within a tubularpassageway, comprising: forming an expandable bistable device with aplurality of bistable cells, each of the bistable cells comprising firstand second arcuate members, the expandable bistable device having agenerally tubular shape; surrounding the expandable bistable device withan expandable liner element attached to an outer surface of the bistabledevice; placing the expandable bistable device at a position within thetubular passageway while in a first stable state; and expanding theexpandable bistable device into a second stable state to hold the linerelement against an inner diameter of the tubular passageway.
 120. Themethod as recited in claim 119, further comprising locating multiplebistable devices in the passageway such that the ends of the adjacentbistable devices overlap and form a continuation of the liner elementagainst the inner diameter of the tubular passageway.
 121. The method asrecited in claim 119, further comprising creating each bistable cell sothat the first arcuate member comprises a thin strut and the secondarcuate member is a thick strut.
 122. A method of isolating a portion ofa passageway, comprising: inserting within the passageway an expandablebistable device having a generally tubular shape formed by a pluralityof bistable cells that permit the expandable bistable device to beselectively actuated between a contracted state and an expanded state,each of the bistable cells comprising first and second arcuate members;and deploying the expandable bistable device to the expanded state toisolate a portion of the passageway.
 123. An apparatus for use in apassageway, comprising: a tubular conduit having at least one bistabledevice, the bistable device comprising first and second arcuate members.124. The apparatus as recited in claim 123, wherein the bistable devicecomprises a plurality of bistable cells, the first and second arcuatemembers of each bistable cell being connected to each other at theirends, the device being stable in a first generally tubular configurationand a second generally tubular configuration, wherein the secondgenerally tubular configuration has a larger diameter than the firstgenerally tubular configuration.
 125. The apparatus as recited in claim124, wherein the apparatus further comprises a device able to transportthe apparatus to a location in the passageway.
 126. The apparatus asrecited in claim 125, wherein the apparatus further comprises adeployment device that initiates the expansion of the bistable device.